In an electromagnetic riveting (EMR) system, such as shown in U.S. Pat. No. 4,862,043 to Peter Zieve, a plurality of high-energy DC capacitors, connected into a parallel arrangement of capacitors, referred to herein as a "bank" of capacitors, are repeatedly charged to a selected voltage, i.e. 500 volts, and then discharged through a flat coil to create a significant repulsion force between the coil and a driver element, which is propelled forwardly to produce a desired work effect, such as forming a rivet or insertion of a bolt, by means of a tool at a nose end of the driver. In one particular embodiment of an electromagnetic riveting system, two separate banks of capacitors are used to power opposing riveters, with the two banks of capacitors being separately charged and discharged.
Such a system must be quite rugged and capable of continuous automatic operation. In many applications, such as the manufacture of aircraft, the output force pulse produced by the riveting apparatus must be carefully controlled and in particular must be matched to the characteristics of the fastener being worked upon. Different fasteners, i.e. different sizes/configurations of rivets and/or bolts, will require somewhat different force pulses. The output force pulse of the riveting apparatus must hence be controllable and produce repeatable results within a relatively close tolerance. Accordingly, the control circuits for the EMR system, in particular the power supply, are a very important part of the system.
Many important operating features of an electromagnetic riveting system are associated in some respect with the operation of and characteristics of the power supply. For instance, the amount of force produced by the system is established by adjusting the voltage present on the capacitor banks before they are discharged. Repeatability of the riveting process requires compensation in the value of the charge voltage taking into account the varying temperature of the riveting coil and the capacitor bank, among other variables. Typically, this variation in charge voltage is accomplished through rather complex control algorithms. The power supply and the system controller must be relatively sophisticated in order to implement such a compensation approach. The accuracy of the charge voltage on the DC capacitor bank must be relatively high, at least in some applications, in order to achieve the desired results. In some cases, the accuracy must be within .+-.one volt. Such accuracy has not heretofore been possible with existing apparatus and techniques.
Further, in the operation of a conventional EMR in a rugged environment, from time to time a chip or other particle from the riveting operation (such as drilling) will actually enter the EMR riveter and penetrate the face of the EMR coil, resulting in arcing and possible destruction of the coil when the driver comes into physical contact with the coil, which occurs once per cycle. Also, the EMR electrical cables are subject to wear, such as in bushings, etc., and electrical shorting or arcing to ground can occur, again leading to system damage. While ground fault indicator (GFI) devices have been used on some electromagnetic riveting systems, these devices are typically powered from the charge voltage on the capacitor banks, such that the GFI device is reliable only when the capacitors are charged. If a chip intrusion does occur with such a system, the capacitor bank voltage (500 VDC) will shift, producing sufficient ringing noise in the wiring to sometimes discharge the capacitor bank (firing the device) at the wrong time, resulting in damage to the workpiece, which is highly undesirable.
Still further, it has been found desirable to efficiently monitor the charge voltage on the DC capacitors to ensure that the capacitors are not being overcharged. In this regard, it is important that the sensing of the charge voltage on the capacitors be independent of the actual operation of the riveting system itself but not require an external source of power.
Lastly, previous electromagnetic riveting apparatus have monitored overall operation of the EMR power system by using current transformers to measure the flow of current from the capacitor bank to the coil. A proper current value is a reliable indicator of proper operation of the EMR power system; however, current transformers are undesirable, since they require a substantial amount of space and a reset winding.